It is well known to smear a liquid on a slide and then subject the liquid to examination under a microscope. This is done widely, for example, with blood to make a white blood cell differential count. The quality of the smear is important. Thus, automatic blood cell differential analyzers cannot provide an accurate white blood cell differential count unless there is an adequate uniform area of blood cell monolayer in the smear. Not only is it important to provide a good smear, but it is also important to provide uniformity of smear from one slide to another.
Prior art devices for smearing a slide are shown in the following patents:
German Pat. No. 439,124--Jan. 24, 1927 PA1 U.s. pat. No. 3,683,850--Aug. 15, 1972 PA1 U.s. pat. No. 3,880,111--Apr. 29, 1975
The device of the German patent is unsatisfactory since it is not possible to control the speed of the spreader while smearing manually to provide either a good smear or smears which are uniform from one smear to the other. While U.S. Pat. Nos. 3,683,850 and 3,880,111 disclose good devices, they lack the highly advantageous features provided by the invention. The prior art fails to teach the mounting of the carriage on a pair of spaced runners which provides for smooth travel of the carriage with low friction and a low rate of wear. The novel spreader which is pivotally mounted and free to rock transversely is advantageous over the prior art since it can accommodate itself to the surface of the slide. While spring and dashpot drive means have been employed as, for example, in U.S. Pat. No. 3,683,850, their use has resulted in considerable vibration being transmitted to the spreader to the detriment of the smear due to the fact that the plunger of the dashpot during the drive portion of the cycle moves from the open end of the dashpot towards the closed end having a restricted opening and hence operates against a column of air causing a flutter or vibration. The invention also provides means to bring the spreader into contact with a liquid deposited on the slide and then move it a short distance away to separate excess liquid from the liquid to be smeared which then moves along the length of the spreader by capillary action. This prevents the spreader from smearing too much liquid on the slide when an excessive amount of liquid is placed on the slide. The prior art also fails to disclose means to support the slide at an angle of at least 5.degree., preferably in the range of 5.degree. to 8.degree., to the plane in which the carriage moves in order to cause a decrease in the angle between the spreader and the slide as the smearing action takes place. While slides have been placed at a slight angle to the plane in which the carriage supporting the spreader moves incident to structures which provide for accommodating slides of different lengths, as in U.S. Pat. No. 3,880,111, the resultant angles were too small to effect any significant decrease in the thickness of the smear. A novel combination of means for advancing the spreader and releasing the spreader for retraction provides for the proper time delay for the liquid to spread along the spreader. It is also known in the art to use a loose fitting spreader on a pin along with restricting sleeves on the pin to permit the spreader to move vertically and rock but to be restricted in its side to side movement. This invention eliminates the complexity of the sleeves by employing restricting side walls adjacent the spreader.